The invention relates to a head of a mould for the hot-top continuous casting of a metallurgical product, such as a steel bloom, billet or slab.
In the case of the continuous casting of a metallurgical product, a molten metal is poured into an upper part or head of a mould having a vertical general disposition and extracted from this mould, via the bottom, is a peripherally solidified product.
The process called xe2x80x9chot-top continuous castingxe2x80x9d, which in fact constitutes an improvement of the general continuous casting process, is used in such a way that the meniscus (the free surface of the cast metal) is transferred upstream of the level where the solidification of the metal inside the head of the mould starts. In order to carry out the hot-top continuous casting process, the usual copper tubular element of the mould, cooled by internal circulation of cooling water, is surmounted, perfectly contiguously, by an uncooled feed head made of thermally insulating refractory material, serving as a reservoir of molten metal fed by the pouring jet from a tundish placed a short distance above it. By virtue of this novel type of construction of the mould head, the liquid-metal meniscus is established therein, during the casting run, within the refractory feed head, whereas the solidification of the metal starts only level with the cooled metal tubular element which, as in conventional continuous casting, calibrates the cast product in terms of shape and size. Consequently, the stirring of the liquid metal due to the pouring jet is limited within the feed head. In the solidification space defined by the copper tubular element placed below, the flow of cast metal may thus be maintained in a relatively calm hydrodynamic state, thereby making it possible in particular to even out the solidification profile of the steel in contact with the cooled copper wall all around the inner perimeter of the mould. However, in order to use such a process satisfactorily, it is necessary to avoid any premature solidification of the cast metal in the feed head so as to be able to ensure that the solidification starts lower down, precisely at the point of contact with the cold copper wall.
To do this, it has already been proposed to leave a gap of very small width (less than 1 mm and generally about 0.2 mm) between the refractory feed head and the copper tubular element and to inject, via this slot, a fluid, generally an inert gas such as argon, into the mould around its inner periphery. In order to ensure gas flow at any point in the slot, the latter is fed with pressurized gas via a distribution chamber which surrounds it.
This injection of gas has the effect of shearing the heterogeneous parasitic solidified film which could form above, against the inner wall of the refractory feed head, and thus create conditions conducive to a sharp and even onset of solidification in the cooled copper element located just below.
In the case of non-circular moulds, in other words in the case of moulds provided with a cooled tubular element quadrangular in shape (for casting slabs, blooms or billets of square cross section, for example) or more generally multiangular in shape (for casting blanks already having the shape of the desired end product), it has been observed, on the cast products after complete solidification, that there are solidification defects along the edges, such as longitudinal cracks, exfoliations, etc., defects whose origin can be identified as being a lack of solidified metal at these points already in the mould, and therefore at the very moment that the solid shell forms.
The object of the present invention is specifically to provide a solution making it possible to reduce, or even to completely eliminate, these solidification defects in the corners of the cast products obtained.
For this purpose, the subject of the invention is a mould for the hot-top continuous casting of molten metals, comprising a cooled metal tubular element of quadrangular shape, defining the shape and size of the cast product and in which the molten metal solidifies on contact with the cooled inner metal wall, the said cooled tubular element being surmounted by an uncooled feed head made of thermally insulating refractory material defining a reservoir of molten metal to be solidified, a slot for injecting a shearing fluid (especially a pressurized inert gas, preferably such as argon) around the inner periphery of the mould being provided between the cooled metal element and the refractory feed head, the said mould being characterized in that it is provided with means for reducing the flow of shearing fluid in the corners.
Preferably, these means consist of an element forming an obstacle to the flow of the gas in the injection slot, the said element being placed in each of the corners of the slot.
The invention results from the following considerations. In order to obtain a satisfactory shearing effect on the flow of gas injected at the base of the feed head, it is necessary to maintain a gas flow rate all along the slot so that there are no dead regions where undesirable solidification fragments would therefore persist. However, even if the slot is fed from a peripheral pressurized-gas manifold, and therefore ensuring that head losses are equal and, consequently, that there is a linear emerging flow with a constant flow rate over the entire length of the slot, an injected-gas flow rate equal at every point around the perimeter of the cast product is not obtained. This is because there is a greater flow rate of gas in the corners of the mould due to the fact that, since the slot is, of course, of the same rectangular shape as the mould, the inside of the latter is fed with gas in two directions in its corner regions. This greater flow rate in the corners results, in the region of the slot, and therefore in particular in the upper part of the cooled copper element located just below, in an overpressure which can cause local separation of the solidified shell from the cold copper wall at the edges of the cast product. It is these separations which, because of the collapse in the effectiveness of the product cooling in the corners which results, are responsible for solidification-disturbing phenomena of the xe2x80x9clack of solidified metalxe2x80x9d type, which phenomena are then manifested, on the cast product obtained, by solidification defects in the corners along the edges.